The present invention relates to the formation of semiconductor devices. More specifically, the present invention relates to etching through a patterned mask.
During semiconductor wafer processing, features of the semiconductor device are defined in the wafer using well-known patterning and etching processes. In these processes, a photoresist (PR) material is deposited on the wafer and then is exposed to light filtered by a reticle. The reticle is generally a glass plate that is patterned with exemplary feature geometries that block light from propagating through the reticle.
After passing through the reticle, the light contacts the surface of the photoresist material. The light changes the chemical composition of the photoresist material such that a developer can remove a portion of the photoresist material. In the case of positive photoresist materials, the exposed regions are removed, and in the case of negative photoresist materials, the unexposed regions are removed. Thereafter, the wafer is etched to remove the underlying material from the areas that are no longer protected by the photoresist material, and thereby define the desired features in the wafer.
Various generations of photoresist are known. Deep ultra violet (DUV) photoresist is exposed by 248 nm light. Presently, for 248 nm photoresist a typical critical dimension (CD) for the photoresist may be 130-250 nm using conventional processes. Due to optical properties dependent on wavelength, photoresist exposed by longer wavelength light has larger theoretical minimal critical dimensions. In order to provide features with smaller CD, features formed using shorter wavelength light are being pursued. 193 nm photoresist is exposed by 193 nm light. Using phase shift reticles and other technology, a 55-100 nm CD photoresist pattern may be formed using 193 nm photoresist. This would be able to provide a feature with a CD of 90-100 nm. 193 nm Immersion photoresist is exposed by 193 nm light with water in direct contact with the wafer surface. Using phase shift reticles and other technology 55 nm CD photoresist patterns may be formed and this will be extended in the future. This would be able to provide a feature with a sub 90 nm CD. EUV (13 nm) photoresist is exposed by 13 nm light. Using this technology sub 22 nm CD photoresist patterns may be formed. This would be able to provide a feature with a sub 55 nm CD.
The use of shorter wavelength photoresists may provide additional problems over photoresists using longer wavelengths. To obtain CD's close to the theoretical limit, the lithography apparatus should be more precise, which would require more expensive lithography equipment. Shorter wavelength photoresists may not have selectivities as high as longer wavelength photoresists and may more easily deform under plasma etch conditions (except for EUV photoresists which typically use 248 nm based backbones for their base polymers).
For photolithography printing, 193 nm immersion scanners for PR patterns have reached their limit in terms of the size of the optics which determines the maxim resolution, i.e., the minimum pattern size they can print. In order to go beyond the optical limit, for example, to achieve a half-pitch pattern, the design pattern may be split into two masks. For example, in a dual line approach, a first PR mask pattern (with a first set of lines) is printed using a first mask, and then a second PR pattern (with a second set of lines) is printed using a second mask. The combination of the first and second sets of lines will reduce the line pitch to a one-half. Such an approach is referred to as “double patterning” or “litho-etch-litho-etch” process. Conventional litho-etch-litho-etch process typically involves etching a hardmask twice: first through the first PR mask and then through the second PR mask. Certain litho-etch-litho-etch processes use two layers of hardmasks; the first hardmask etched through the first PR mask; and the second hardmask selectively etched through the second PR mask. In either case, the first PR mask is stripped before the second PR mask is formed.
An alternative approach of double patterning uses a protective layer for a first PR mask, which is formed before applying a second PR material onto the first PR mask. In general, when a liquid PR material is applied onto a wafer having a patterned PR mask, the polymer of the patterned PR mask would dissolve when it comes in contact with most organic solvents. Thus, another type of formulated system, such as a water soluble, acid cross-linkable coating material, can be used to form a protective layer onto the first PR mask so as to prevent the patterned PR mask from dissolving into an organic solvent of the second PR material. It is preferable that the solvent of the protective coating material does not dissolve the first PR so that the coating does not disturb the first patterned PR significantly. Suitable solvents may be water, fluorosolvents, silicon solvents, or polar solvents like methanol, ethanol or other similar alcohols. When a water-soluble, acid cross-linkable protective layer is applied on to the first PR mask and baked, the water is driven out and the residual amount of acid comes out from the first PR mask to its surface. Since the coating material is acid cross-linkable, a cross-linked polymer coat is formed on the surface of the first PR mask. Then, the coating material that is not cross-linked can be washed away, leaving the first PR pattern with the cross-linked polymer coat. This process may be referred to as “chemical freeze” of the first PR mask pattern as it “freezes” the shape of the first PR mask.
However, although it is water soluble polymer, the cross-linked protective layer coat still has affinity with the organic solvent. Thus, when the second PR material is liquid-applied on top of the coated first PR mask, the organic solvent of the liquid PR material causes the cross-linked area to form a “gel” and the first PR pattern swells and/or deforms, which in turn causes line-edge roughness (LER), line distortion and/or line lifting and defects. In addition, in order to preserve the original critical dimension (CD), it is preferable that the cross-linked coat on the first PR pattern is as thin as possible, but this exacerbates the deformation problems